Fiber type composition affects the speed of a muscle contraction, but less so the specific tension (force per unit area). Force depends not only on the size and number of the fibers, but also on muscle architecture. Because the maximal force per unit of cross-sectional area (specific tension) of skeletal muscle is considered relatively constant (approx. 22.5 N/cm2
) (Lucas et al., 1987
; Maganaris et al., 2001
), contractile force of a skeletal muscle can be estimated based on its physiological cross-sectional area (PCSA) (Lieber & Friden, 2000
), represented by the equation:
where M is muscle mass, θ represents the angle of the fibers (pennation), ρ is muscle density (1.056 g/cm3
in mammalian muscle) and Lf
represents fiber length (estimated from length of the measured fiber bundle). Based on this equation we estimate that the pyramidalis muscles in this subject would generate minimal force, in fact, <1% of the estimated force generated by a rectus abdominus in the normal sized male (Rankin et al., 2006
). For example, for the right pyramidalis muscle the mass was 1.75 g, the fibers had 0° angulation (cosine of 0 = 1), density is 1.056 g/cm3
(or 0.01056 g/mm3
), and the mean Lf
was 37 mm. Therefore, if the PCSA is 1.75 × 1/0.01 × 37 = 4.7 mm2
, then the estimated tension generated by one pyramidalis muscle (0.225 N/mm2
× 4.7 mm2
) would be only approximately 1 N. The relative importance that this modest amount of force has on the linea alba is not clear.
The microscopic and ultrastructural characteristics of skeletal muscle have been described in detail, and it is established that muscle is a highly organized tissue at the cell level. But the arrangement of muscle fibers and muscles themselves has received less scrutiny. This arrangement of muscle fibers (muscle architecture) and the location of muscles are important parameters because they both clearly affect how muscles function (Lieber & Friden, 2000
). For example, a muscle with long fibers will have a fast shortening speed and increased excursion (moving any joints it crosses through a greater range of motion) compared to a muscle of equal length, but with shorter muscle fibers. The faster shortening speed and increased excursion are due to the accumulative effect of many sarcomeres arranged in series. PCSA, which takes into account variations in mass, fiber length, and arrangement of fibers within the muscle, is directly proportional to the maximal tetanic tension that a muscle can generate (Lieber & Friden, 2000
). In addition to muscle architecture, an important variable that determines function is the attachment of a muscle relative to the axis of movement. The moment arm resulting from the attachment of the pyramidalis muscles is likely to be unimportant because the forces generated by these muscles suggest that they are not prime movers of the lumbar spine, or any other joint.
The purpose of the present case report was to identify the fiber type and architecture of the paired pyramidalis muscles, located at the base of the rectus sheath. We have shown that these muscles, at least in the present subject, have a fiber type composition that is similar to the rectus abdominus and that the fiber length of a single pyramidalis muscle is varied from medial (longest) to lateral (shortest). Because the cadaver was already perfuse-fixed, we were not able to control for sarcomere length; but even if the muscle were fixed in a slightly shortened or lengthened position, it is unlikely that a change in fiber length would contribute a significant change to the relatively small forces generated by this muscle. We do not know if the lack of pennation (0°) is typical of most pyramidalis muscles, but it is also unlikely that minor variation of fiber direction would affect force generation by this small muscle. For example, even a change from 0° (cosine = 1) to 20° (cosine = 0.9) would potentially reduce the PCSA (with all other variables remaining the same), but only by <10%. Therefore, a change in pennation angle is not likely to effect force production in this small muscle. The small forces (approx. 1 N each side) are consistent with the concept that the pyramidalis muscles are used mainly to develop tension in the rectus sheath, rather than as prime movers of a joint.
Even if the present findings can be extrapolated, they might be limited to an older population. Magnetic resonance imaging would be a useful tool to examine the incidence and size of the pyramidalis in younger populations, and a post-mortem study with a large sample size would be warranted to confirm our observations concerning the architecture and fiber type composition of the pyramidalis muscles.